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DEPARTMENT   OP   COMMERCE 


Technologic  Papers 


OF  THE 


Bureau  of  Standards 

S.  W.  STRATTON,  DIRECTOR 


No.  157 

AN  INVESTIGATION 

OF  THE  PHYSICAL  PROPERTIES  OF 

DENTAL  MATERIALS 


WILMER  H.  SOUDER,  Associate  Physicist 
CHAUNCEY  G.  PETERS,  Associate  Physicist 

Bureau  of  Standjirds 


ISSUED  MAY  22,  1920 


PRICE,  10  CENTS 

Sold  only  by  the  Superintendent  of  Documents,  Government  Printing  Offica 
Washington,  D.  C. 


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DEPARTMENT   OF    COMMERCE 


Technologic  Papers 


OF   THE 


Bureau  of  Standards 

S.  W.  STRATTON,   DIRECTOR 


No.  157 

AN  INVESTIGATION 

OF  THE  PHYSICAL  PROPERTIES  OF 

DENTAL  MATERIALS 


BY 


WILMER  H.  SOUDER,  Associate  Physicist 
CHAUNCEY  G.  PETERS,  Associate  Physicist 
Bureau  of  Standards 


ISSUED  MAY  22,  1920 


PRICE.  10  CENTS 

Sold  only  by  the  Superintendent  of  Documents,  Government  Printing  Office 
Washington,  D.  C. 


WASHINGTON 
GOVERNMENT  PRINTING  OFFICE 
1920  • 


So  i 


AN  INVESTIGATION  OF  THE  PHYSICAL  PROPERTIES 
OF  DENTAL  MATERIALS 


By  Wilmer  H.  Souder  and  Chauncey  G.  Peters 


CONTENTS 

Page 

I.  Introduction 3 

I.  Historical 4 

II.  Instruments 5 

1.  The  Black  micrometer  and  Wedelstaedt  tube 5 

2 .  The  Black  dynamometer 6 

3.  The  flow  attachment 7 

4.  Optical  micrometers  and  comparators 7 

III.  Manipulation  of  alloy .' 8 

IV.  Dimensional  changes  with  temperature 9 

1 .  Apparatus  and  methods 9 

2.  Experimental  procedure 15 

3.  Results 16 

V.  Dimensional  changes  with  time 20 

1 .  Apparatus 20 

2.  Experimental  procedure 21 

3.  Results 21 

VI.  Flow  under  compression 30 

VII.  Crushing  strength 34 

VIII.  Blackening  of  hand 35 

IX.  Chemical  composition 35 

X.  Electrode-potential  determinations 37 

XI.  Summary 39 

I.  INTRODUCTION 

A  recent  request  from  a  branch  of  the  Government,  purchasing 
large  quantities  of  dental  supplies,  resulted  in  the  authors  taking 
up  a  systematic  study  of  the  physical  properties  of  certain  filling 
materials  together  with  the  instruments  generally  used  in  testing 
the  same. 

Properties,  such  as  crushing  strength,  flow,  thermal  and  chemical 
expansions,  chemical  composition,  electrode-potentials  and  thermal 
reactions  have  been  investigated.  Additional  phases,  such  as 
method  of  manipulation,  time,  temperature,  etc.,  were  found  to 
exert  definite  influences  in  certain  tests  and  indicated  the  necessity 
for  their  proper  control. 

3 


4  Technologic  Papers  of  the  Bureau  of  Standards 

It  is  believed  that  the  discoveries  in  instruments  will  be  of 
sufficient  importance  to  justify  lengthy  descriptions  since  it  was 
found  necessary  to  discard  practically  all  devices  described  in  the 
dental  text  books.  In  the  selection  of  instruments  we  have  con- 
sidered two  features,  viz,  accuracy  and  simplicity. 

Some  of  the  alloys  included  in  this  research  were  made  to 
requested  specifications,  others  were  purchased  in  the  open  market, 
and  many  were  submitted  by  manufacturers  cooperating  in  the 
investigation. 

Many  of  the  results  should  be  interpreted  as  comparative  or 
relative  tests.  Since  it  is  not  our  purpose  to  advertise  the  merits 
or  demerits  of  any  material,  the  manufacturer's  names  have  been 
omitted.  Bach  has,  however,  been  informed  of  the  results  found 
for  his  alloy.  The  purpose  of  this  paper,  as  in  all  previous  reports, 
is  to  place  before  those  familiar  with  the  use  of  dental  materials 
accurate  and  reliable  data,  together  with  a  description  of  instru- 
ments suitable  for  measuring  the  properties  investigated. 

We  have  attempted  to  discuss  some  of  our  results  on  a  purely 
physical  basis.  Their  clinical  interpretation  is  left  to  those 
experienced  in  the  profession. 

It  is  hoped  that  at  a  near  date  and  with  the  cooperation  of  the 
manufacturers  and  users  of  these  materials  it  will  be  possible  to 
write  definite  and  proper  specifications,  which  will  enable  the 
purchaser  and  user  to  secure  articles  of  known  qualities. 

1.  HISTORICAL 

It  is  practically  impossible  to  give  a  complete  bibliography  of 
the  researches  on  amalgam  during  its  75  years  of  existence. 

The  first  amalgams  were  manipulated  in  a  crude  and  empirical 
way,  thus  entailing  an  unusually  large  number  of  failures.  In 
fact,  the  feeling  against  amalgam  was  so  strong  at  one  time  that 
any  one  speaking  favorably  of  its  possibilities  was  immediately 
boycotted  by  the  profession.  Is  it  any  wonder  that  failures  were 
very  conspicuous  when  we  read  of  the  crude  practices  and  lack  of 
definite  data  on  the  manufacture  and  manipulation  of  alloys? 
Opinions  and  speculations  seem  to  have  been  considered  of  more 
importance  than  data  or  accurate  statistics. 

Despite  the  failures  the  few  successful  restorations  were  so 
completely  satisfactory  that 'investigators  started  a  search  to  find 
the  essential  properties  and  technique  incident  to  satisfactory 
restorations.     The  works  of  the  pioneers,  Tomes,  Fletcher,  Hitch- 


Physical  Properties  of  Dental  Materials  5 

cock,  Witzel,  and  Flagg,  have  been  reviewed  and  elaborated  by- 
Black,  who  has  probably  done  most  to  point  out  the  necessity  for 
accurate  scientific  tests.  The  chapter  on  amalgams  in  Volume  II 
of  Black's  Operative  Dentistry  (19 14)  contains  a  world  of  infor- 
mation on  early  work,  together  with  the  results  of  his  own  extensive 
researches. 

More  recent  laboratory  researches  are  recorded  in  the  various 
dental  journals  imder  the  authorship  of  Marcus  L.  Ward,  dean, 
U.  of  M.  Dental  School;  A.  W.  Gray,  research  director,  h.  D. 
Caulk  Co. ;  W,  G.  Crandal,  Spencer,  Iowa;  C.  M.  McCauley,  Abilene, 
Tex.;  William  B.  Harper,  Chicago,  111.;  A.  Fenchel,  Hambiurg, 
Germany;  B.  R.  Bakker,  Utrecht,  Holland;  and  McBain  &  Joyner, 
Bristol,  England, 

The  findings  of  these  men  are  not  concordant  in  all  essentials 
and  the  lack  of  definite  descriptions  of  instruments  and  manipula- 
tive details  makes  it  difficult  to  explain  all  the  results  reported. 
It  is  hoped  that  the  present  article  may  clear  up  some  of  the  points 
in  question  among  the  various  investigators  and  also  enable  those 
interested  in  continuing  the  work  to  proceed  with  apparatus  of 
unquestionable  accuracy  and  simplicity. 

II.  INSTRUMENTS 

The  first  instruments  of  importance  for  systematically  testing 
amalgams  were  those  of  Dr.  Black's  design — the  micrometer  and 
dynamometer.  These  were  of  the  utmost  value  in  seciuring  quali- 
tative information  and  started  a  new  era  in  amalgam  manufacture, 
but  they  can  scarcely  be  relied  upon  to  give  the  most  precise  or 
decisive  data.  Few,  if  any,  investigators  are  using  them  to-day. 
The  writers  are  familiar  with  the  apparatus  used  by  Gray  at  Milf ord 
and  by  Ward  at  Michigan  and  feel  sure  that  both  types  are  an 
advance  over  the  original  apparatus  of  Black  and  quite  accurate 
for  their  researches. 

1.  THE  BLACK  MICROMETER  AND  WEDELSTAEDT  TUBES 

Fimdamental  errors  precluding  the  use  of  the  Black  micrometer 
for  precision  work  are  the  mechanical  impossibilities  (to  date)  of 
combining  a  system  of  levers  and  gears  to  operate  a  dial  or  mirror 
indicator  wliich  will  accurately  or  consistently  measure  directly  to 

inch  (2^  microns).     Scale  divisions  on  an  instrument  are 


10  000 

not  always  an  indication  of  its  japproximate  sensitivity,  although 


6  Technologic  Papers  of  the  Bureau  of  Standards 

they  should  be.  The  elements  of  variance,  passivity,  and  backlash 
always  enter  where  there  is  a  multiplicity  of  moving  parts.  These 
features,  as  affecting  instrument  design,  are  fully  treated  in  an 
article  by  F.  J.  Schlink,  Scientific  Paper  No.  328  of  this  Bureau. 

The  Wedelstaedt  tubes  used  with  this  micrometer  are  subject 
to  criticism  because  of  their  restraining  action  on  the  amalgam 
specimen.  Later  it  will  be  seen  that,  as  shown  below,  the  expan- 
sion of  amalgam  is  about  two  and  one-half  times  as  large  as  steel; 
hence  on  the  least  temperature  rise  the  amalgam  is  frictionally 
locked  against  the  sides  of  the  tube  and  in  many  cases  the  excess 
expansion  causes  a  bulging  or  "  spheroiding "  of  its  free  surface. 
The  same  phenomenon  will  occur  with  chemical  or  crystallization 
expansions.  Any  later  temperature  drop  will  be  accompanied  by 
the  larger  contraction  of  the  amalgam  (see  section  on  thermal 
expansion) ,  thus  giving  the  ' '  black-ditch  "  effect.  A  slight  heating 
of  one  of  these  tubes  of  amalgam  is  sufficient  to  cause  an  apparent 
excessive  expansion  or  spheroiding  followed  by  the  ditch  effect. 

Again  the  "points"  expansion  are  not  in  the  same  units  as 
"points"  contraction,  since  in  the  latter  case  the  amalgam  may 
shrink  in  all  dimensions;  whereas  in  the  former  the  three  dimen- 
sions of  expansion  have  been  forced  into  a  threefold  one  dimension 
of  expansion,  two  of  which  are  now  manifest  as  flow,  but  recorded 
as  expansion.  This  probably  accounts  for  the  claim  of  slight 
expansion  and  no  contraction  which  is  often  made  for  alloys; 
whereas  in  fact  there  is  contraction  which  is  too  small  for  detection 
with  such  apparatus. 

2.  THE  BLACK  DYNAMOMETER 

Little  need  be  said  about  the  dynamometer  except  that  the 
results  are  of  necessity  irregular,  due  to  the  smallness  of  specimens. 
This  smallness  of  specimen  permits  undue  influence  from  slight 
variations  of  manipulation,  mixing,  or  packing.  Cubical  forms 
are  not  well  suited  for  these  crushing  tests.  The  exposed  corners 
and  equal  dimensions  are  factors  which  should  be  avoided.  For 
the  proper  test  of  compressibiHty,  as  outlined  by  the  American 
Society  for  Testing  Materials,  specimens  should  be  prepared  in 
cylindrical  form,  i  inch  in  diameter  and  2>^  to  4  inches  long. 
Special  attention  is  directed  to  the  necessity  of  using  specimens  with 
a  length  greater  than  the  diameter  in  order  to  avoid  the  "barrel" 
effect  in  case  there  is  a  tendency  toward  flow. 


Bureau  of  Standards  Technologic  Paper  No.  157 


i^,.      ..  3^     %-~*,tt. 


Photo  by  R.  E.  Lofton,  mag.  X  S5 

Fig.  I. — Amalgam-enamel  7nar gin 

Same  margin  viewed  by  different  illuminations.     Wrong  interpretations  often  result  from  an  improper 

use  of  the  microscope 


Physical  Properties  of  Dental  Materials  7 

3.  THE  FLOW  ATTACHMENT 

Some  types  of  dynamometers  are  equipped  with  an  auxiliary 
flow  dial  which  records  the  motion  of  the  compression  rod  as  the 
pressure  is  applied  for  crushing.  The  objections  offered  above 
apply  equally  well  against  this  attachment  as  an  instrument  of 
precision. 

The  sudden  shock  given  the  levers  and  dial  parts  at  the  instant 
of  rupture  is  sufficient  to  disturb  the  bearing  surfaces  and  adjust- 
ments of  all  parts  of  the  instrument. 

4.  OPTICAL  MICROMETERS  AND   COMPARATORS 

Unless  handled  by  a  person  skilled  in  their  use  these  instru- 
ments are  of  little  value.  The  accompanying  figure  is  included  to 
explain  the  error  possibilities  of  devices  depending  upon  micro- 
scopes; (a)  and  (&),  Fig.  i,  are  photographs  of  the  same  tooth, 
with  absolutely  no  changes  of  position  of  microscope  or  tooth,  the 
only  change  being  a  slight  modification  of  the  illumination.  In 
the  first  instance  there  is  no  question  about  the  imperfect  adapta- 
tion of  amalgam  to  tooth;  in  the  second  it  appears  reasonably 
perfect.  The  differences  are  even  more  striking  when  using  the 
binocular  microscope. 

Micrometer  microscopes  are  often  used  in  comparing  and  cali- 
brating line  (length)  standards.  These,  when  arranged  for  the 
best  possible  accuracy — that  is,  best  illumination  and  most  suit- 
able lines  on  a  properly  surfaced  background — will  give  results 
agreeing  within  i  micron.  However,  if  only  a  single  microscope 
is  used  and  the  displacement  measured  in  terms  of  the  run  of  a 
screw,  settings  having  been  made  with  the  assistance  of  a  micro- 
scope, then  it  is  necessary  to  add  to  the  uncertainties  of  the  micro- 
scopic setting  the  errors  and  irregularities  of  the  screw.  A  third 
source  of  error  lies  in  the  necessity  of  using  an  auxiliary  contact  to 
transmit  any  motion  of  the  amalgam,  since  the  new  amalgam 
surface  is  constantly  changing  its  character  and  necessarily  also 
the  character  or  appearance  of  any  line  or  mark  which  may  have 
been  placed  thereon. 

The  combined  errors  make  it  difficult,  if  not  impossible,  to  detect 
variations  of  i  or  2  microns,  and  again  the  slight  contractions  of 
samples  in  Wedelstaedt  tubes  may  pass  unnoticed. 

Optical  lever  devices  read  by  the  mirror  and  scale  method  are 
only  slightly,  if  any,  better.  Their  use  necessitates  a  multiplicity 
of  contacts  and  bearing  surfaces,  each  of  which  is  subject  to  its 


8  Technologic  Papers  of  the  Bureau  of  Standards 

own  peculiar  action,  and  each  introduces  its  inherent  source  of 
error. 

The  safer  way  to  proceed  in  using  any  of  the  above  instruments 
is  to  use  samples  of  amalgam  of  larger  dimensions,  thus  magni- 
fying the  effect  and  consequently  increasing  the  accuracy  of  values. 

Probably  the  best  instrument  for  rough  measurements  is  an 
ordinary  small  screw  machinist  micrometer  adjusted  for  a  speci- 
men about  50  mm  long.  This  instrument  is  reliable  to  better  than 
-^  mm  (3T5in5-  inch) ,  about  2  so-called  Black  points;  but  the  speci- 
men being  about  six  times  as  long  as  the  Wedelstaedt  specimen 
there  results  a  net  increase  in  theoretical  accuracy.  Such  an  out- 
fit properly  handled  in  a  thermostated  chamber  will  be  of  more 
decisive  value  than  many  of  the  above-described  instruments, 
which  are  used  in  looking  for  ditches  or  bulges  and  depend  entirely 
upon  the  magnification  of  margins. 

III.  MANIPULATION    OF   ALLOY 

This  subject  is  often  lost  sight  of  in  the  discussion  of  physical 
properties.  Probably  no  other  phase  of  the  restoration  is  subject 
to  greater  carelessness.  The  dentist  may  spend  unusual  care  in 
shaping  the  cavity  after  having  removed  a  generous  amount  of 
good  tooth  structure  to  make  siu"e  the  last  traces  of  decay  have 
been  eliminated  and  then  proceed  to  make  up  the  amalgam  on  the 
assumption  that  the  only  requirement  is  to  daub  up  the  cavity. 
Or  if  the  assistant  is  doing  the  amalgamation  and  starts  too  soon, 
he  is  instructed  to  keep  working  the  mass  until  the  cavity  is  ready. 
Another  practice  is  to  keep  the  mass  plastic  by  the  addition  of 
slight  amounts  of  mercury  at  intervals,  thus  making  it  possible  to 
condense  several  cavities  from  the  same  mix.  The  serious  effects 
of  this  over  trituration  will  be  found  in  another  section. 

The  amalgamation  and  condensation  procedure  throughout  this 
research  (unless  otherwise  specified)  was  according  to  manufac- 
turers' instructions.  In  the  absence  of  definite  instructions  the 
following  technique  was  adopted  and  is  not  seriously  different 
from  the  majority  of  manufacturers'  instructions. 

The  alloy  and  mercury  were  weighed  in  an  approximate  ratio, 
such  that  there  would  be  a  very  slight  excess  of  mercury  on  con- 
densation. These  were  mixed  (not  ground)  in  a  mortar  for  one 
minute.  The  amalgamated  mass  was  then  transferred  to  the 
hand  and  mulled  two  minutes.  Part  of  the  excess  mercury  was 
removed  through  chamois  cloth.  The  artificial  or  matrix  cavities 
were  holes  in  steel  blocks  drilled  and  polished.     The  amalgam  was 


Physical  Properties  of  Dental  Materials  9 

condensed  in  these,  using  three  or  four  different-sized  pluggers. 
Those  specimens  which  failed  to  show  perfect  adaptation  to  the 
form  were  rejected.  Sufficient  alloy  was  used  so  that  an  excess  of 
amalgam  could  be  built  up  over  the  top  of  the  cavity,  and  on  tap- 
ping lightly  it  was  usually  possible  to  cause  additional  mercury  to 
rise  from  the  cavity.  This  excess  mercury  and  amalgam  was  later 
removed  and  the  specimen  taken  from  the  matrix.  The  conden- 
sation was  usually  completed  in  two  or  three  minutes,  depending 
upon  the  size  and  shape  of  specimen. 

The  sUght  excess  of  mercury  used  appeared  to  give  the  con- 
densed amalgam  a  greater  uniformity  and  prevent  the  forma- 
tion of  layers,  when  added  quantities  of  amalgam  were  con- 
densed in  the  cavity.  These  layers  are  fostered  by  delay  in 
condensing,  hence  the  desirability  of  using  shorter  times. 

The  greatest  variation  in  results,  when  using  a  given  manipula- 
tion, appears  to  arise  not  so  much  from  slight  irregularities  in 
manipulation  of  alloy  as  from  variation  in  alloy  from  package  to 
package.  All  values  reported  for  alloy  G  ^  are  for  the  same  pack- 
age; this  is  also  true  for  alloy  I.  A  fair  degree  of  agreement 
will  be  found  for  check  tests  on  these  alloys  recorded  in  Table  3. 
Another  method  of  securing  uniformity  of  results  resorted  to  by 
some  investigators  consists  in  packing  specimens  under  con- 
tinued mechanical  pressure,  allowing  several  minutes  to  elapse 
before  removing  them  from  the  matrix.  The  authors  have  been 
more  interested  in  finding  what  happens  to  amalgams  as  they 
are  ordinarily  used,  or  should  be  used,  and  have  made  an  at- 
tempt to  start  tests  early  enough  to  discover  all  changes  ac- 
companying the  reaction,  regardless  of  variations. 

IV.  DIMENSIONAL   CHANGES    WITH   TEMPERATURE 

1.  APPARATUS  AND  METHODS 

For  the  measurement  of  the  thermal  expansion  of  small  sam- 
ples, methods  and  apparatus  which  make  use  of  the  interference 
of  light  waves  have  been  employed  at  this  Bureau  for  several 
years.  For  some  of  this  work  the  old-established  Fizeau^  method 
was  used.  Fig.  3  shows  an  interferometer  devised  by  Priest^  for 
determining  the  thermal  expansion  of  single  small  pins.     With 

'  Letters  have  been  assigned  to  the  trade  names  of  the  alloys  and  throughout  this  paper  the  same  letter 
refers  to  the  same  alloy,  but  not  necessarily  to  the  same  package  of  alloy  except  in  the  cases  mentioned 
above. 

'  Pizeau,  Annal  de  Chem.  et  al  Phy.  (4)  2,  143;  1:863. 

•  Priest,  B.  S.  Scientific  Paper  No.  365,  1930. 

1605X0°— 20 2 


lo  Technologic  Papers  of  the  Bureau  of  Standards 

another  form  of  apparatus,  recently  described  by  one  of  the 
authors/  the  expansion  of  three  different  pins  can  be  simultane- 
ously determined.  These  interferometers  have  been  thoroughly 
tried  out  and  found  to  measure  small  displacements  with  an  error 
of  about  0.005  micron  or  0.0000002  inch.  Because  of  the  ex- 
treme accuracy  and  directness  of  the  method  and  the  necessity  of 
using  small  samples,  the  interferometer  was  used  for  the  de- 
terminations of  thermal  expansion  of  teeth  and  dental  materials. 
A  rather  complete  description  of  the  apparatus  has  been  given 
here  for  those  who  may  be  interested  in  making  measurements 
of  this  kind. 

The  interferometer  is  usually  thought  of  as  an  extremely  com- 
plicated apparatus,  while  in  reality  the  essential  parts  are  two 


Fig.  2. — Vertical  section  of  apparatus 

plane  glass  plates  which  are  held  apart  by  a  suitable  separator. 
Fig.  2  shows  a  vertical  section  of  the  apparatus. 

The  two  plates  A  and  B  and  the  ring  D  constitute  the  inter- 
ferometer, while  P  represents  a  Pulfrich  apparatus  for  viewing 
the  fringes. 

The  light  from  a  helium  lamp  H  is  focused  upon  a  small  total 
reflection  prism  p.  After  being  collimated  by  the  lens  Oi,  it  is 
reflected  by  the  prism  R  down  to  the  interferometer  plates  A  and 
B  which  are  in  the  focal  plane  of  O^.  The  rays  returning  from 
points  in  the  plane  of  the  mirrors  are  colHmated  by  the  lens  O^, 
and  an  image  of  the  interference  pattern  and  the  reference  marks 
on  the  plates  is  formed  by  the  lens  O2  upon  the  sUt  5  and  viewed 

,  *  Peters,  Jour.  Wash.  Acad,  of  Sci.,  9,  p.  281;  1919. 


Physical  Properties  of  Dental  Materials 


II 


with  the  eye  piece  C.     The  direct  vision  prism  K  separates  the 
fringe  patterns  due  to  the  helium  light  of  different  wave  lengths. 

A  more  detailed  view  of  the  interferometer  is  shown  in  Fig.  3. 
The  upper  surface  of  the  base  plate  P  is  a  polished  true  plane. 
One  side  of  the  plate  which  projects  about  0.3  mm  above  the 
upper  surface  of  B  forms  a  knife  edge  55  parallel  to  that  surface, 
while  the  other  side  is  undercut,  leaving  the  edge  EF  parallel  to 
55,  and  the  base  M  which  forms  a  support  for  the  sample  x. 


^M 


Fig.  3. — Interferometer 


The  upper  end  of  the  sample  is  cut  away  to  the  center  from  one 
side  and  bevelled  to  an  edge  from  the  other.  The  flat  surface  of 
the  sample  is  placed  in  contact  with  the  edge  EF,  thus  assuring  a 
constant  distance  D  between  the  sample  and  the  knife  edge  55. 
Two  reference  lines  H  and  K,  distance  d  apart,  are  ruled  parallel 
to  the  knife  edge  55  on  the  upper  surface  of  the  base  plate. 

The  upper  interferometer  mirror  A,  which  is  a  plate  of  glass 
with  both  faces  polished  true  plane,  rests  on  the  knife  edge  55 


1 2  Technologic  Papers  of  the  Bureau  of  Standards 

and  the  top  of  the  sample  x,  adjusted  to  be  slightly  higher  than 
S5.  A  narrow  wedge-shaped  space  is  thus  formed  between  the 
two  plates  the  widest  part  of  which  is  toward  x.  Considering  the 
two  faces  of  this  wedge,  light  from  the  lamp  H  made  parallel  by 
the  lens  0^,  Fig.  2,  falling  upon  the  lower  surface  of  A,  Fig.  3, 
is  in  part  reflected  and  the  rest  transmitted  to  the  upper  surface 
of  B.  Here  again  part  of  the  light  is  reflected.  Between  these 
two  reflected  wave  trains  "interference"  takes  place.  The 
observer,  viewing  this  reflected  light,  sees  straight  dark  bands 
parallel  to  each  other  across  the  face  of  the  wedge.  In  Fig.  3, 
/i>  f2i  fzi  ^tc,  represent  these  bands.  The  band  /^  shows  that 
along  that  line  the  distance  down  and  back  between  the  plates 
is  some  whole  number  of  wave  lengths.  On  moving  to  a  wider 
part  of  the  wedge  another  line  /a  is  reached  where  twice  the  dis- 
tance between  the  plates  is  one  wave  length  greater  than  at  f^. 
Similarly  along  /g  this  distance  is  two  wave  lengths  greater  than 
at  /i,  etc.  Starting  from  /^  each  successive  band  denotes  that  the 
separation  of  the  two  plates  has  increased  by  one-half  the  wave 

I 

length  of  the  light  or  about  of  an  inch.     Therefore  the 

100  000 

total  number  of  bands  between  55  and  x  multiplied  by  one-half 
the  wave  length  of  the  light  used  i-\  gives  the  difference  in  sepa- 
ration of  the  plates  at  x  and  S5  or  the  difference  in  elevation  of  x 
and  SS  above  the  upper  siuiace  oi  B.  As  it  is  difficult  to 
determine  the  number  of  bands  between  x  and  S5,  the  number 
N  between  the  two  reference  lines  H  and  K  is  determined.  The 
corresponding  distances  D  and  d  are  known,  therefore  the  differ- 
ence in  elevation  of  x  and  5S  above  B  at  any  time  is  equal  to 
\D  N 

2  d 

IvCt  Lx  denote  the  length  of  the  sample  and  Ls  the  length  of 
standard  material  between  S5  and  the  plane  of  M.  Let  AL^ 
and  ALs  represent  the  elongation  of  Lx  and  Ls  caused  by  a  rise 
of  temperature  AT.  If  L^  and  Ls  expand  differently  the  relative 
elevations  of  x  and  55  change,  causing  a  change  in  the  number 
of  bands  between  H  and  K.  Let  N^  represent  the  number  of 
bands  between  the  reference  marks  before  and  N^  the  number 
after  the  temperature  change  takes  place;  then, 

AL.=  AU  +  ^{N,-N,)  (i) 


Physical  Properties  of  Dental  Materials  13 

That  is,  the  elongation  of  x  is  equal  to  the  elongation  of  s  plus 
the  difference  in  elongation  of  the  two,  v/hich  is  determined  from 
the  change  in  the  number  of  bands  between  the  reference  marks. 
The  coefficient  of  linear  thermal  expansion  is  the  elongation  of 
unit  length  per  degree  rise  of  temperature 

L  AT 

The  coefficient  of  expansion  of  x  is  then  given  by  the  expression 

r         AL.  ALs       \D  {N,  -  N,)       ,  . 

"~LxAT"L.Ar^     2dUAT  ^^^ 

Since  Lx  and  Ls  are  Dractically  equal  this  becomes 
C.  =  C.  +  ^|^f^  •  (4) 

2(2   LxAl 

Where  Cs  is  the  coefficient  of  expansion  of  the  base  plate 
material. 

In  order  to  control  the  temperature  properly  the  interferometer 
was  mounted  on  a  steel  block  G  in  the  bottom  of  the  container 
C,  Fig.  4,  which  consisted  of  a  steel  tube,  30  cm  long,  5  cm  in 
diameter,  and  i  mm  thickness  of  v/all,  the  upper  end  of  which 
was  closed  with  a  glass  window  W^;  10  cm  from  the  base  another 
glass  v/indow  W^  was  supported  by  a  heavy  brass  ring.  Most  of 
the  lower  part  of  the  tube  was  cut  av/ay  to  allow  easy  adjustment 
of  the  interferometer.  This  end  of  the  tube  was  closed  with  a 
steel  cup  R  which  screwed  onto  the  tube  at  U  with  a  rubber 
gasket  to  make  the  joint  tight.  A  little  merctuy  in  the  bottom 
of  the  cup  made  good  metallic  contact  between  the  base  block 
and  the  bath.  The  container  v\^as  lowered  with  a  rack  and 
pinion  into  the  oil  bath  which  regulated  the  temperature.  The 
bath  liquid  was  circulated  through  the  tubes  D  and  E  by  the 
propeller  P.  To  eliminate  vibration  the  motor  M  was  supported 
on  a  separate  base  and  connected  to  the  propeller  shaft  by  a 
small  rubber  tube.  The  liquid  was  cooled  by  the  brine  coil  K 
and  heated  with  a  lo-ohm  resistance  coil  H  the  current  in  which 
was  regulated  by  a  relay  operated  by  the  thermostat  /.  This 
apparatus  was  thermally  insulated  with  ground  cork  and  mounted 
in  a  wooden  box. 

The  temperature  of  the  liquid  around  C  could  be  held  for  any 
length  of  time  within  0.01°  of  the  desired  value.     The  tempera- 


14 


Technologic  Papers  of  the  Bureau  of  Standards 


ture  of  the  bath  was  read  with  a  thermometer  T  and  a  five- 
junction  thermocouple  L,  and  the  temperature  of  the  base  block 
and  sample  with  another  five- junction  thermocouple  N.  A 
temperature  survey  was  made  of  this  apparatus  with  differential 


Fig.  4. — Temberahire  control  apparatus 

thermocouples  to  determine  both  the  time  required  for  the 
interferometer  and  sample  to  reach  a  steady  state  when  the 
temperature  of  the  bath  was  held  constant,  and  the  lag  of  the 
sample  when  the  bath  temperature  was  changing. 


Physical  Properties  of  Dental  Materials 


15 


2.  EXPERIMENTAL  PROCEDURE 

The  amalgam  material  after  being  prepared  in  the  manner 
already  described  was  made  into  samples  about  6  mm  in  diameter 
and  I  cm  long,  illustrated  by  x,  Fig.  3.  These  samples  were  kept 
for  several  weeks  before  measurements  were  made.  The  porcelain 
samples  were  made  in  the  same  form  and  kept  under  water.  The 
tooth  specimens  were  taken  from  different  teeth  and  different 
parts  of  the  same  tooth.  One  of  the  samples,  the  expansion  of 
which  is  shown  in  Fig.  5,  was  taken  from  the  crown  of  a  large 
molar,  the  other  from  the  root  of  a  long  cuspid. 
5. — \ 1 r 


20 


30 


40 


50     60    70 
DEGREES  CENTIGRADE 


80 


Fig.  5. — Thermal  expansion  of  teeth 
(I=crowii.    II=root) 

After  the  sample  was  properly  adjusted  the  interferometer 
was  placed  in  the  container  and  lowered  into  the  bath.  Two 
procedures  were  followed  in  making  the  measurements.  The 
first  was  to  hold  the  temperature  of  sample  constant  for  at  least 
one  hour  at  each  point  before  the  measurements  were  made. 
The  second  was  to  change  the  temperature  of  the  sample  slowly, 
about   1°  C  in  three  or  four  minutes,   and  make  observations 

periodically. 

With  the  samples  from  teeth  it  was  found  that  as  soon  as  they 
were  heated  moisture  evaporated  and  a  very  rapid  contraction 


i6 


Technologic  Papers  of  the  Bureau  of  Standards 


took  place  amounting  to  0.2  to  0.3  mm  for  a  i-cm  sample.  These 
samples  regained  their  original  length  if  allowed  to  stand  in  water 
for  two  or  three  days.  To  overcome  this  contraction,  which  was 
many  times  greater  than  the  thermal  expansion  of  the  material, 
the  interferometer  and  sample  were  placed  in  a  container  filled 
with  water.  This  container  was  then  placed  in  the  temperatm-e- 
control  apparatus.  With  both  the  porcelain  and  teeth  it  was 
found  necessary  to  keep  the  samples  under  water  and  make  the 
measurement  with  the  samples  under  water  in  order  to  obtain 


50  6  0  7  0  80 

DEGREES   CENTIGRADE 

Fig.  6. — Thermal  expansion  of  synthetic  porcelain 

consistent  results.  This  procedure  seems  to  reproduce  actual 
condition  of  the  mouth  and  yield  values  of  thermal  expansion  as 
near  correct  as  it  is  possible  to  obtain  with  this  type  of  material. 

3.  RESULTS 

The  following  curves  represent  the  elongation  of  the  different 
materials  with  change  of  temperature.  Degrees  centigrade  are 
plotted  as  abscissae  and  change  in  length  (AL)  in  microns  (ix) 
of  a  sample  i  cm  long  as  ordinates.  All  the  observations  that 
were  taken  have  been  plotted,  none  were  rejected  or  omitted. 


Physical  Properties  of  Dental  Materials 


17 


Curve  /,  Fig.  5,  is  for  a  sample  cut  from  the  crown  of  a  molar, 
Curve  //  for  a  sample  from  the  root  of  a  newly  extracted  cuspid. 

Curves  /  and  //,  Fig.  6,  represent  the  expansions  of  two  different 
samples  of  porcelain. 

The  curves  in  Figs.  7  and  8  are  for  different  kinds  of  amalgams. 
These  curves  are  lettered  to  correspond  with  kind  of  material 
from  which  the  samples  were  made.  In  fact,  most  of  the  samples 
had  previously  been  used  for  experiments  of  setting  changes. 

Most  of  the  samples  showed  irregular  behavior  near  80°  C. 
Some  of  the  samples  when  removed  from  the  container  were 


100  130 

OSNTIGSIADS 

Fig.  7. — Thermal  expansion  of  amalgam 

covered  with  soft  drops  of  mercury  which  formed  into  bright 
crystals  in  a  short  time.  An  inspection  of  these  curves  and  the 
samples  showed  that  the  amalgams  must  have  undergone  some 
radical  transformations  when  heated.  It  is  very  probable  that 
any  filling  or  part  of  a  filling,  if  by  chance  subjected  to  tempera- 
tures near  this  value,  may  suffer  serious  injury  or  have  its  physical 
properties  entirely  changed. 

In  Table   i   there  are  tabulated  data  on  the  Hnear  thermal 
expansion  of  a  number  of  materials,  including  sections  of  teeth. 
160510°— 20 3 


i8 


Technologic  Papers  of  the  Bureau  of  Standards 


From  this  table  it  is  seen  that  the  expansions  of  teeth  range  from 
6.4  to  1 1.4,  depending  upon  the  tooth  selected  and  the  portion 
of  the  tooth  used,  an  average  value  of  approximately  8.  The 
porcelain  average  value  is  not  far  from  the  above  figure,  gold  is 
somewhat  higher  (14.4),  while  the  amalgams  average  about  25. 


40 

' — 

B 

6 

S  30 

/ 

/ 

/ 

0 
^  20 

^A 

L 

Je5 

J 

y 

^ 

y 

^  10 

ri^ 

t 

K 

^ 

^ 

9' 

y 

a 

-.^ 

^ 

1^ 

/ 

20  40  60  80  100  120 

DE(31EES  CENTIGRADE 

Fig.  8. — Thermal  expansion  of  amalgams 
TABLE  1.— Average  Expansion  Coeflacients,  Range  20  to  50°  C 


Material 


Tooth  (root) 

Tooth  (across  crown) . . 
Tooth  (root  and  crown) 

Do 

Do 

Synthetic  porcelain 

Do 

Do 

Amalgam  (H) 

Amalgam  (C) 

Amalgam  (K) 

Amalgam  (P) 

Amalgam  (A) 

Amalgam  (B) 

Amalgam  (L) 


Expansion 
coefficients 

X  10  8 


8.3 

H.4 

6.4 

8.7 

8.3 

7.1 

8.1 

7.5 

26.4 

25.0 

22.1 

24.5 

25.4 

28.0 

24.8 


Material 


Amalgam  (C) 

do 

do 

Porcelain  (Bayeus) 

Gold 

Platinum 

Silver 

Mercury  (linear) . . 

Zinc 

Tin 

Copper 

Gutta-percha 

Aluminum 

Steel 


Expansion 
coefficients 

Xl0  6 


25.0 
24.7 
28.0 

4.1 
14.4 

9.0 
19.2 
60.6 
29.2 
22.3 
16.8 
198.3 
23.1 
11.0 


Physical  Properties  of  Dental  Materials  19 

Since  the  effects  due  to  the  differential  expansions  of  the 
materials  depend  upon  the  temperature  range,  size  of  cavity, 
and  elasticity  of  tooth  substance  and  material,  no  general  state- 
ment should  be  made  regarding  the  relative  merits  of  materials 
unless  all  the  important  conditions  are  given  due  consideration. 

To  illustrate,  let  us  consider  the  case  of  a  maximum  cavity  and 
filling  I  cm  in  diameter,  undergoing  a  temperature  variation  of 
50°  C.  Then  the  free  expansion  along  each  coordinate  axis  is  4 
microns  for  the  cavity,  7  microns  for  the  gold  filUng,  and  12.5 
microns  for  the  amalgam.  (If  the  dimension  or  temperature 
range  is  less,  these  effects  will  be  proportionally  reduced.)  If 
we  have  perfect  adaptation  and  no  stress  at  the  lower  temperature, 
then  at  the  higher  temperattu-e  there  are  two  extreme  possibili- 
ties: {a)  The  elasticity  of  the  tooth  and  compressibility  of  the 
material  may  be  such  that  perfect  adaptation  is  maintained,  or 
(6)  the  rigidity  of  the  tooth  and  plasticity  of  the  material  may  be 
such  that  there  will  be  a  flow  of  material  in  the  only  free  direction 
causing  a  spheroiding  or  bulging  over  the  cavity.  "With  a  perfectly 
rigid  tooth  cavity  this  may  equal  three  times  the  linear  differential 
expansion  of  the  substances,  which  will  be  25  microns  in  the  case 
of  amalgam.  Should  the  filling  material  take  a  permanent  set  at 
the  higher  temperature,  then  on  returning  to  the  lower  tempera- 
ture, all  materials  having  imdergone  free  contraction,  there  is  a 
possibility  of  a  4-micron  separation  around  the  filling.  This  is 
analagous  to  the  case  of  heating  the  Wedelstaedt  tube  referred  to  in 
a  previous  section. 

The  above  temperature  range  of  50°  C  has  been  decided  upon 
as  a  fair  representation  of  the  temperature  variation  to  which 
metallic  fillings  may  be  subjected.  In  order  to  test  this  experi- 
mentally, copper-constantan  thermocouples  were  cemented  into 
amalgam  inserts  i  to  3  mm  from  the  surface,  and  the  temperatiu-es 
of  the  inserts  were  read  by  means  of  a  potentiometer  when  different 
foods  and  drinks  were  taken  into  the  mouth.  With  ice  water  and 
cracked  ice  temperatures  as  low  as  5°  C  and  with  hot  foods  and 
drinks  temperatures  from  50  to  60°  C  were  observed  in  the  inserts. 
No  unusual  or  excessive  sensation  of  pain  was  indicated  by  the 
adjacent  vital  teeth  during  the  experiments. 


^o  Technologic  Papers  of  the  Bureau  of  Standards 

V.  DIMENSIONAL   CHANGES    WITH   TIME 
1.  APPARATUS 

The  apparatus  described  in  the  previous  section  was  used  to 
measure  the  dimensional  changes  of  newly  made  amalgam  samples 
during  the  hardening  or  setting  period.  The  temperature  of  the 
sample  and  interferometer  was  held  constant  and  the  variations  in 
the  length  of  the  specimens  with  time  were  recorded.  These 
dimensional  variations  during  hardening  will  be  referred  to  as 
setting  changes. 

In  equation  (i),  ALs  represents  the  change  in  the  length  of 
the  material  under  the  knife-edge,  due  to  the  change  in  tempera- 
ture. In  the  present  case  the  temperature  was  constant,  there- 
fore ALs  equals  zero  and  the  expression  for  the  setting  change  of 
the  sample  becomes 

AL.  =  ^^(iV,-iV,)  (5) 

Here  the  interferometer  becomes  a  lever  of  the  second  class  with 
the  knife-edge  forming  the  fulcrum,  the  top  of  the  sample  the 
point  of  application  of  the  force,  and  the  upper  plate,  which 
weighs  about  lo  grams,  the  load.  This  type  of  lever  is  far  less 
susceptible  to  friction  and  contact  error  than  those  used  in  the 
moving  dial  or  mirror  instruments.  The  only  restraining  force 
on  the  sample  is  the  constant  5  grams  weight  of  the  cover  plate. 

The  results  obtained  from  several  samples  made  from  the  same 
material  and  held  constant  at  temperatures  ranging  from  8  to 
37°  C  showed  that  careful  temperature  control  was  not  of  suffi- 
cient importance  to  warrant  the  use  of  the  constant  temperature 
apparatus. 

Four  interferometers  of  different  construction  were  mounted  in 
a  semicircle  on  a  table  so  that  the  Pulfrich  apparatus  could  be 
turned  from  one  to  another  for  making  observations  on  the  inter- 
ference fringes.  A  thermometer  was  placed  close  tO;  the  inter- 
ferometers and  the  temperature  of  the  room  was  recorded  after 
each  observation  on  the  fringes.  This  arrangement  greatly 
facilitated  the  work,  for  four  samples  were  investigated  simul- 
taneously, whereas  the  constant  temperature  apparatus  acconmiQ- 
dated  only  one  at  a  time. 


Physical  Properties  of  Dental  Materials  21 

2.  EXPERIMENTAL  PROCEDURE 

The  alloys  were  amalgamated  and  condensed  into  the  mold 
according  to  the  procedm-e  previously  described.  The  specimens 
were  immediately  removed  from  the  mold  and  adjusted  to  fit  the 
interferometer,  which  required  from  4  to  8  minutes.  The  cover 
plate  was  pressed  down  upon  the  sample  with  a  force  of  several 
pounds  to  insure  good  contact  at  the  bearing  points  and  to  ascer- 
tain whether  or  not  the  sample  had  hardened  enough  to  support 
the  weight  of  the  plate.  Most  of  the  samples  hardened  in  a  few 
minutes,  but  a  fev/  Hke  M  and  N,  Fig.  14,  were  still  quite  soft 
after  60  minutes.  The  time  record  started  the  moment  the 
sample  was  removed  from  the  mold  and  observations  of  the  length 
changes,  which  commenced  immediately  after  the  adjustment  of 
the  samples,  were  taken  at  given  intervals  over  a  period  of  one  to 
several  days.  With  rapidly  changing  materials  of  this  kind  the 
number  of  interference  bands  between  the  reference  lines  was 
estimated  to  the  nearest  o.i  of  a  band.  Since  0.2  of  a  band  is 
equivalent  to  about  o.oSju  this  estimation  gave  all  the  accuracy 
necessary. 

At  least  three  samples  from  each  material  were  investigated 
according  to  the  above  procedure. 

3.  RESULTS 

The  following  curves  represent  length  changes  which  took 
place  dining  setting.  Time  in  minutes  is  plotted  as  abscissae 
and  change  of  length  (AL)  in  microns  (m)  of  a  sample  i  cm  long  as 
ordinates.  An  ascending  curve  represents  an  elongation  and  a 
descending  curve  a  contraction.  The  time  required  to  adjust 
the  sample  to  the  interferometer  is  plotted  along  the  zero  ordi- 
nate. The  mean  temperatiure  and  maximum  variation  in  tem- 
perature of  each  sample  have  been  recorded  with  the  curve. 

In  Fig.  9  are  represented  the  setting  changes  of  four  samples 
from  material  C.  The  samples  were  all  prepared  according  to 
our  regular  procedmre  but  were  held  at  different  temperatures 
after  being  placed  in  the  interferometer.  The  temperatures  at 
which  the  different  samples  were  held  during  the  runs  are  as 
follows:  Sample  C-i  at  37°  C,  C-2  at  27.8°  C,  C-3  at  2p.4°  C, 
and  C-4  at  8°  C. 

Fig.  10  represents  the  data  taken  on  four  samples  of  material  B. 
The  samples  were  prepared  according  to  our  regular  manipulation 
and  held  very  near  to  the  same  temperature  throughout  the  ex- 


22 


Technologic  Papers  of  the  Bureau  of  Standards 


6 


8  10  13 

HUNIBED  MINUTES 


Fig.  9. — Setting  changes  of  amalgam  C 
Spedmens  held  at  different  temperatures 


u 


8  10         13 

HUNDRED  MINUTES 


14 


Fig.  10. — Setting  changes  of  amalgam  B 
Specimens  held  at  nearly  the  same  temperature 


Physical  Properties  of  Dental  Materials 


23 


periments.  The  mean  temperature  of  the  sample  and  the  maxi- 
mum variation  during  the  run  are  recorded  with  each  curve. 
Figs.  II  to  13  represent  the  results  obtained  from  samples  of 
alloys  C,  H,  and  A,  which  had  been  given  various  treatments 
before  and  subjected  to  similar  conditions  after  condensation. 
The  mulHng  time  was  varied  from  i  to  35  minutes.  A  specimen 
of  each  alloy  was  cooked  for  three  hours  at  120°  C,  and  i^ 
per  cent  zinc  was  added  to  one  alloy. 

From  the  foregoing  curves  it  is  seen  that  the  length  of  time 
devoted  to  mulHng  the  amalgam  exerted  by  far  the  greatest  effect 
upon  its  subsequent  behavior.     The  curves  from  samples  of  mate- 


4 

/, 

'r 

;S 

^ 

— 

2^ 

TTfc 

S        ° 

a 

J 

y 

y 

f 

82! 

^ 

L 

%  -13 

I 

\ 

-16 

^ 

C-4 

... 

izi. 

\rc 

' 

■ 

4 


8  10  12         14 

HUNDRED  MINUTES 


Fig.  II. — Setting  changes  of  amalgam  C 

C-i,  Usual  manipulation;  C-2,  alloy  cooked  3  hours  at  120°  C;  C-3,  mulled  i  minute;  C-4,  m.uUed  35  minutes 

rial  B,  Fig.  lo,  which  received  practically  the  same  treatment,  while 
quite  concordant,  do  not  show  a  much  better  agreement  than  those 
of  material  C,  Fig.  9,  which  were  held  at  different  temperatures 
dining  the  setting  period.  In  Figs.  11,  12,  and  13  curve  i  rep- 
resents the  usual  manipulation.  Cinve  2,  in  which  alloys  were 
cooked  for  three  horn's  at  120°  C,  differs  very  little  from  curve  i, 
C-3  and  H-3,  which  were  mulled  only  one  minute,  agree  almost  as 
well  with  curve  i .  The  addition  of  i  ^  per  cent  zinc  to  alloy  A 
seems  to  have  changed  the  character  of  the  setting  curve  A-j. 
The  marked  effect  caused  by  mulling  the  amalgams  20  to  35 


24 


Technologic  Papers  of  the  Bureau  of  Standards 


minutes  is  shown  by  curve  4.  Experiments  of  this  kind  showed 
that  large  setting  contractions  could  be  produced  in  any  of  the 
amalgams  by  overmulling  and  emphasized  the  importance  of  that 
part  of  the  process.  Compared  with  this  effect  the  slight  irregu- 
larities due  to  variations  in  room  temperature  seem  to  be  insig- 
nificant. If  setting  contractions  of  10  to  20/i  per  centimeter  are 
important,  then  this  question  of  mulling  time  must  be  given  careful 
consideration. 


Fig.  12. 


6  8  10  12 

HUNDRED  MlhTJTES 

-Setting  changes  of  amalgam  H 


H-i,  Usual  manipulation;   H-2,  alloy  cooked  3  hours  at  120°  C;   H-3,  mulled  i  minute;   H-4,  mulled  25 

minutes 

Oui  results  upon  effects  of  mulling  agree  very  well  with  the  find- 
ings of  Dr.  Gray .5  He  has  placed  greater  emphasis  upon  the 
importance  of  accurate  temperature  control  of  the  samples  during 
the  setting  period  than  we  have.  Regarding  the  effect  of  annealing 
no  comparison  can  be  made  because  he  has  neglected  to  state  the 
temperature  at  which  the  annealing  was  carried  out. 

The  setting  changes  of  each  of  the  other  materials  are  shown 
in  Figs.  14  to  17.     At  least  three  samples  from  each  material  pre- 

'  Gray,  Journal  of  the  National  Dental  Association,  6,  10,  p.  917;  1919. 


Physical  Properties  of  Dental  Materials 


25 


6  8  10  12  14 

HUNDRED  MIITUl-ES 
Fig.  13. — Setting  changes  of  amalgam  A 


A-i,  Usual  manipulation;  A-2,  alloy  cooked  3  hours  at  120°  C;  A-3,  i%  Zn  added  to  alloy;  A-4,  mulled  20 

minutes 


W 
n 

o 


4  6  8  10  12  14 

HUNDRED     MINUTES 

Fig.  14. — Setting  changes  of  amalgams 


26 


Technologic  Papers  of  the  Bureau  of  Standards 


pared  according  to  the  regular  procedure  were  tested  and  gave 
concordant  characteristic  curves  similar  to  those  shown  for  mate- 
rial C,  Fig.  9. 

Only  one  curve  from  each  material  has  been  plotted  for  the  pur- 
pose of  making  a  comparison  of  tfie  behaviors  of  the  different 
material. 

From  the  chemical  analysis,  Table  3,  it  will  be  seen  that  M  and 
N,  Fig.  14,  are  low  silver  alloys.  These  specimens  were  very  slow 
in  setting;  in  fact,  it  was  impossible  to  make  any  measurements 
during  the  first  hour  because  the  material  would  not  support  the 
lightest  pressure.     Alloy  S  contained  73  per  cent  silver.     Alloy  P 


•4 


u 

-3 

1 

• 

/ 

.^^ 

' — 

-^^ 

-4 

/ 

/ 

£ 

Z2S 

*.€< 

-6 

n^" 

/ 

If^ 

r 

f 

— 

— 



-8 

V 

\ 

A 

I 

zs:o 

^ 

\. 

-10 

"^ 

-*-, 

_£__ 

23J) 

tic 

4  6  8  10  12  14  16 

HUNDRED  MIITDTES 

Fig.  15. — Setting  changes  of  amalgams 

contained  5  per  cent  zinc.  Alloys  E,  I,  and  F  are  not  widely 
different  in  setting  changes  during  the  first  hour,  but  diverge 
somewhat  after  this  time.  All  show  permanent  shrinkage. 
Alloys  B,  G,  and  T  of  Fig.  16  have  practically  the  same  silver 
analyses.  Alloys  K  and  L,  Fig.  1 7 ,  are  lower  in  silver  than  E  or  F. 
The  difference  in  behavior  is  probably  due  to  relative  percentages 
of  two  other  elements. 

Nearly  all  of  the  samples  show  an  initial  contraction.  With 
some  this  continued  and  no  recovery  was  evidenced;  with  others 
an  initial  contraction  of  from  i  to  3  microns  took  place  in  the  first 
30  or  40  minutes,  after  which  the  sample  expanded  for  the  next 
40Q  minutes,  remaining  quite  constant  during  the  remainder  of 


Physical  Properties  of  Dental  Materials 


27 


the  experiment.  The  initial  contraction  could  be  due  to  one  or  all 
of  at  least  three  causes.  The  sample  is  usually  quite  soft  for  a  fev/ 
minutes,  which  might  cause  a  settling  of  the  sample  under  even 
its  own  weight;  there  might  be  a  real  contraction  of  the  material; 
or  in  adjusting  the  sample  it  is  heated  somewhat  in  the  hand,  which 
is  slightly  above  the  room  temperature.  In  the  last  case  a  thermal 
contraction  should  take  place  during  the  first  few  minutes. 

To  follow  the  actual  temperatures  of  the  sample  during  the 
setting  or  hardening  period,  one  junction  of  a  differential  thermo- 
couple was  placed  in  a  block  which  had  the  same  temperature  as  the 


10     13 

HUNDRED  MINUTES 


14 


16 


Fig.  16. — Setting  changes  of  amalgams 


room.     The  other  junction  was  packed  in  the  center  of  the  sample, 
which  was  made  and  adjusted  according  to  the  usual  procedure. 

Measiirements  made  immediately  after  samples  were  placed  in 
the  interferometer  showed  that  their  temperatures  were  4  to  8°  C 
above  the  temperature  of  the  block  which  was  very  nearly  22.5°  C 
during  these  experiments.  Fig.  18  shows  the  cooling  curves  of 
some  of  these  samples.  The  sample  ip-A  was  3.5°  C  above  the 
temperature  of  the  block  when  placed  in  the  apparatus  and  was 
still  0.7°  C  above  the  temperature  of  the  block  150  minutes  later. 
Three  hoiurs  later  it  was  warmed  8°  C  above  block  temperature 
and  allowed  to  cool.  Curve  iq-B  shows  that  it  returned  to  the 
block  temperature  after  22  minutes. 


28 


Technologic  Papers  of  the  Bureau  of  Standards 


The  second  sample  was  7.4°  C  above  block  temperature  imme- 
diately after  being  put  into  the  apparatus.  Its  cooling  is  shown 
by  curve  16-A.  It  was  still  0.5°  C  above  block  temperature  60 
minutes  later.  The  following  day  its  temperature  was  increased 
5°  C.     It  then  cooled  in  12  minutes  as  shown  by  curve  16-B. 

Curve  4  shows  another  sample  that  required  80  minutes  to 
return  from  5°  C  above  to  block  temperature. 

These  experiments  show  that  heat  is  evolved  by  the  material 
during  the  amalgamation.  This  evolution  c  f  heat  seems  to  con- 
tinue during  the  period  while  the  sample  is  undergoing  most  of 
its  variation  of  length.     Of  course  some  of  the  original  heating 


O 


►3 
<3 


-  3 


10     12     14 
HUNDRED  MINUTES 


16 


Fig.  17. — Setting  changes  of  amalgams 


comes  from  the  hand  of  the  operator,  but  curves  19-B  and  16-B 
show  that  the  sample  should  return  to  the  temperature  of  the 
room  in  15  to  20  minutes.  Enough  heat  seems  to  be  evolved  to 
keep  the  temperature  of  the  sample  above  that  of  the  surround- 
ings for  several  hours.  From  this  it  seems  that  careful  control 
of  the  sample  container  does  not  insure  that  temperature  of  the 
sample  is  the  same  as  that  of  its  surroundings.  Consideration  of 
the  foregoing  results  make  it  possible  to  account  for  i  to  2  microns 
of  this  initial  setting  contraction  by  the  thermal  contraction  of 
the  material.  The  fact  that  some  of  the  samples,  which  hardened 
in  a  few  minutes  after  condensing,  contracted  more  than  that 


Physical  Properties  of  Dental  Materials 


29 


w 
o 


M 
O 

O 
W 


p 

6 

' 

4 

3 

\\ 

K 

\ 

^ 

■^^ 

— . 

■ 

/S-yf 

0 

\ 

— =^ 

Inn 

-"**-= 

r:r 

./tf'4 

— ™ 

M-^ 

/^^ 

90 

30 


40  60  80  100         130         140 

MINUTES 

Fig.  18. — Cooling  of  amalgams  during  and  after  setting 


100  120  140  160 

MimJTES 


Fig.  ig.— Setting  changes  of  synthetic  porcelain  in  air 


30 


Technologic  Papers  of  the  Bureau  of  Standards 


amount  leads  us  to  believe  that  part  of  the  initial  contraction  was 
real. 

The  porcelain  samples  contracted  rapidly  when  exposed  to  the 
air.  This  is  shown  by  curve  T-i  and  T-2,  Fig.  19.  Sample  T-3 
retained  its  length  for  25  minutes  while  under  water  and  then 
contracted  rapidly  when  exposed  to  the  air.  Curve  T-4  repre- 
sents the  behavior  of  a  sample  coated  with  its  varnish  and  exposed 
to  the  air. 

The  curves  in  Fig.  20  represent  the  behavior  of  samples  of  por- 
celain which  were  kept  under  water  during  the  experiment. 
Sample  T-5  was  exposed  to  the  air  during  adjustment  and  dried 


4 

1 

a 

r 

N, 

( 1 
< 

S 

^ 

.^^ 

8  0 

//S 

~-- 



T-S 

-^ 

S 

''-s,* 

\ 

^^ 

k 

^, 

■ ■ 

->^ 

^ 

--. 

^ 

T-6 

•4 

■ 



^*-^ 

T-7 

^^~^ 

■*- 

-6 

1 

2  4  6  8  10  12  14 

HUNDRED  MINUTES 

Fig.  20. — Setting  changes  of  synthetic  porcelain  in  water 


16 


slightly.  Upon  being  put  into  the  water  it  evidently  absorbed 
some  water  and  expanded  2.4  microns  during  the  first  few  min- 
utes; after  that  it  contracted  slowly. 

The  two  samples  T-6  and  T-7  were  kept  moist  while  being 
adjusted  to  fit  the  apparatus  and  show  a  slow  uniform  contraction. 

VI.  FLOW   UNDER   COMPRESSION 

The  authors  have  found  all  amalgams  yielding  under  constant 
pressure  even  months  after  amalgamation.  Hence  the  questions 
of  setting  and  crushing  strength  become  relative  factors.     When 


Bureau  of  Standards  Technologic  Paper  No.  157 


Fig.  21. — Micrometer  adapted  to  measure  flow  of  amalgam 


Physical  Properties  of  Dental  Materials 


31 


the  crushing  load  is  applied  as  rapidly  as  possible  the  crushing 
strength  always  runs  high. 

Most  amalgams,  after  setting  48  hours,  will  show  crushing 
strength  of  over  32  000  pounds  per  square  inch  if  crushed  quickly, 
say  in  3  minutes;  but  by  applying  a  constant  pressure  of  3200 
pounds,  only  one-tenth  the  previous  crushing  load,  we  found  it 
possible  to  crush  some  of  these  same  amalgams  within  20  hours. 

With  these  facts  established  it  was  decided  to  make  compara- 
tive flow  tests,  beginning  2  hours  after  packing,  and  applying  the 
approximate  one-tenth  load,  namely,  3200  pounds  per  square  inch. 


o 

M 
03 

m 

o 
o 


4  6  8  10  13  14  16 

HUNDRED  MimjTES 
Fig.  22. — Flow  of  amalgam;  load  applied  two  hours  after  packing 

The  load  was  applied  to  a  specimen  4  mm  in  diameter  and  about 
8  mm  long. 

The  apparatus  used  for  the  flow  tests  is  shown  in  Fig.  21  and 
consists  of  a  micrometer  with  a  weight  pan  attached  at  the  top 
of  the  upright  rod  or  plunger.  The  two  ends  of  the  specimen  are 
cut  at  right  angles  to  the  axis  and  placed  between  the  jaws  of  the 
micrometer.  As  the  amalgam  is  compressed  by  the  weight  ap- 
plied at  the  top  of  the  rod  the  indicator  moves  around  the  dial. 
The  difference  of  readings  gives  the  amount  of  compression  or 
flow.     An  air  cushion  plunger  incorporated  in  this  micrometer 


32 


Technologic  Papers  of  the  Bureau  of  Standards 


prevents  injury  to  the  instrument  should  the  specimens  fracture 
suddenly. 

Changes  in  length  are  indicated  on  the  dial.  These  are  re- 
corded as  per  cent  change  in  total  length.  Figs.  22  to  25  are 
included  to  portray  the  average  behavior  of  amalgams  when 
subjected  to  this  test. 

Fig.  22  was  plotted  from  data  taken  under  the  above  condi- 
tions. Two  hours  after  amalgamation  and  packing  the  specimen 
was  placed  in  the  compression  micrometer  and  a  pressure  of  3200 
pounds  per  square  inch  applied.     In  30  minutes  it  was  compressed 


o 


O 

o 


i 


2 
4 
6 

1 

\ 

P 

\ 

■ 

10 

\ 

^_ 

F 

6 


8 


10  12  14 

HUNDRED  MINUTES 
Fig.  23. — Flow  of  amalgam;  load  applied  two  hours  after  packing 


16 


40  per  cent.  This  specimen  did  not  flow  materially  after  10  hours, 
due  to  the  fact  that  the  cylinder  had  been  ruptured  and  was 
spread  over  a  much  larger  area,  thus  reducing  the  effective  pres- 
sure per  square  inch. 

There  was  a  possibility  of  this  apparent  recovery  in  ability  to 
resist  flow  being  attributable  to  delayed  crystallization  (slow 
setting),  so  additional  specimens  were  prepared  and  retained  48 
hours  before  being  subjected  to  the  flow  test.  The  same  amal- 
gam under  this  treatment  was  compressed  18  per  cent  and  frac- 
tured after  three  days.     (See  Fig.  25^".) 

Fig.  23  is  plotted  for  another  amalgam,  compression  started 
2  hours  after  packing.     This  alloy  was  marked  "Quick  setting." 


Physical  Properties  of  Dental  Materials 


33 


Fig.  24  (top)  indicates  the  flow  of  a  5  per  cent  Zn  amalgam  and 
the  curve  at  the  bottom  is  for  a  nonzinc  amalgam.  The  amalgams 
in  Fig.  25  were  not  subjected  to  the  one-tenth  load  until  48 
hours  after  condensing.  Amalgam  N  is  the  same  as  that  used  in 
Fig.  22. 

Most  amalgams  v>dll  withstand  this  test  remarkably  well  at 
I  hour  and  some  at  30  minutes  after  packing.  The  qualities 
producing  failure  seem  to  be  inherent  and  permanent  in  the  ma- 
terial (probably  chemical  compositions)  and  permit  failures  after 
48  hours  as  readily  as  after  30  minutes;  the  so-called  slow-setting 
or  quick-setting  qualities  being  difficult  to  interpret. 


o 
ra 

CO 

I 

o 

» 

A* 


3 

v^ 

4 

Sfc^Zn 

6 


8 


10  13  14  16 

HUNDRED  MINUTES 

Fig.  24. — Flow  of  amalgams;  load  applied  two  hours  after  packing 

The  above  may  appear  to  be  a  new  feature,  but  such  is  not  the 
case,  as  it  is  simply  a  modification  or  improvement  on  the  Black 
dynamometer  in  which  a  set  of  levers  operating  a  dial  hand  is  used 
to  indicate  the  compression  during  the  crushing  tests.  The  con- 
stant, continuous  load  of  one-tenth  the  crushing  value  is  selected 
as  appropriate  for  deciding  upon  the  merits  of  an  amalgam  when 
used  in  a  cavity  such  that  it  is  required  to  furnish  a  contact  point 
with  an  adjacent  tooth  or  filling  where  constant  pressure  is  applied. 
The  permanence  in  shape  and  position  of  this  contact  point  will 
eliminate  later  troubles  and  constant  annoyances  due  to  food  par- 


34 


Technologic  Papers  of  the  Bureau  of  Standards 


tides  which  tend  to  wedge  between  the  teeth.  The  possibilities 
of  position  changes  are  of  importance  and  the  inevitable  upsetting 
of  the  amalgam  tooth  margins  needs  only  mentioning  to  bring  one 
to  reahze  the  gravity  of  such  situations.  The  point  of  a  tooth 
from  an  opposite  jaw  constantly  striking  an  amalgam  may  apply 
an  effective  load  of  several  thousand  pounds  per  square  inch 
even  though  the  gross  pressure  is  only  a  few  pounds.  The  small- 
ness  of  the  area  of  contact  has  the  effect  of  intensifying  the  im- 
pacts when  considered  in  terms  of  pounds  per  square  inch.  The 
material  is  being  treated  in  a  manner  very  similar  to  that 
employed  in  battering  the  head  of  a  rivet  where  light  taps  con- 
tinued for  a  short  time  are  quite  sufficient  to  change  the  entire 
cylindrical  end  into  a  flat  bur. 


o 
o 

a 


0 

"^ 

i^         -^ 

\==dU 

-* 

A— «- 

— « 

o 

4 

8 
13 

\     ^ 

^ 

\ 

^^^ 

c 

X^ 

"^--^ 

0 

'^ 

"^W^ 

16 

^^>v^ 

"*<^ 

20 

^ 

--. 

13545678 

DAYS 

Fig.  25. — Flow  of  amalgams;  load  applied  48  hours  after  packing 

VII.  CRUSHING   STRENGTH 

As  previously  stated  crushing  strengths  have  been  found  de- 
pendent upon  the  time  occupied  for  the  test.  The  tests  included 
in  this  report  were  made  as  rapidly  as  convenient,  using  a  speci- 
men 6  mm  in  diameter  and  lo  to  12  mm  long.  While  not  of  the 
size  specified  by  the  A,  S.  T.  M.  mentioned  previously,  they  are  a 
close  approximation  to  the  form.  The  preparation  of  specimens 
I  inch  in  diameter  and  2}4  inches  long  would  introduce  factors  and 
difficulties  of  trituration  and  condensation  which  are  never  en- 
countered in  practice,  to  say  nothing  of  the  enormous  expense 
necessary  to  produce  such  quantities  of  alloy. 


Physical  Properties  of  Dental  Materials  35 

The  regular  testing  machine  equipment  of  the  Bureau  was 
used  for  these  tests.  The  time  required  for  each  crush  was  about 
three  minutes.  Tests  were  made  14  days  after  amalgamation. 
All  materials  included  in  this  test  were  made  up  according  to  the 
regular  dental  technique,  in  fact,  extra  specimens  were  made  up 
by  a  practicing  dentist  for  comparison  and  concordant  results 
were  found. 

By  mechanically  packing  the  specimens  under  high  pressure, 
immediately  after  amalgamation,  it  is  possible  to  secure  crushing 
tests  of  almost  twice  the  values  given  in  the  table  at  the  close. 

The  results  of  compressional  tests  are  somev/hat  irregular,  but 
no  more  so  than  those  found  by  other  observers.  It  is  felt  that 
the  comparisons  between  alloys  makes  these  data  worth  pre- 
senting. 

Since  these  tests  were  for  comparison  and  were  made  under  sim- 
ilar conditions  no  extra  precautions  v/ere  taken  to  control  tem- 
perature, which  was  constant  at  about  25°  C,  probably  within  2°. 

VIII.  BLACKENING   OF   HAND 

This  test  consisted  in  mulling  specimens  in  the  hand  two 
minutes  after  having  first  been  amalgamated  for  one  minute  in 
the'  mortar.  The  hands  were  first  washed,  rinsed  several  times, 
and  then  dried  to  make  sure  that  tests  were  comparable.  A 
zero  value  indicates  that  no  blackening  was  detectable;  10  repre- 
sents a  complete  and  dense  black  coat  or  film  left  in  the  palm. 
The  values  given  are  the  average  of  several  tests  by  different 
manipulators.  In  no  case  was  there  a  variation  of  over  two 
points,  due  to  different  tests  or  observers. 

The  authors  will  not  attempt  to  pass  on  the  merits  of  this  test. 
The  discoloration  of  the  hand  does  not  a  priori  imply  a  discolor- 
ation of  the  tooth  tubuli.  The  free  sulphides  or  combinations 
of  foreign  materials  producing  this  blackening  require  further 
study  to  determine  their  true  effect  in  amalgam  usages. 

IX.  CHEMICAL   COMPOSITION 

The  chemical  compositions  vary  from  45  to  69  per  cent  silver 
and  from  o  to  5  per  cent  zinc,  copper  and  tin  making  up  the 
remainder. 

For  practically  all  alloys  the  claim  of  balance  according  to 
the  G.  V.  Black  standard  is  made,  which  can  probably  be  inter- 
preted to  mean  that  the  resultant  filling  should  expand  slightly, 
never  contract,  and  after  a  few  hours  "  lay  still. " 


36  Technologic  Papers  of  the  Bureau  of  Standards 

While  absolutely  exact  percentages  should  produce  exactly 
the  same  alloys  if  similarly  treated,  it  is  very  probable  that  other 
factors  enter  which  are  far  more  significant;  for  example,  a 
certain  alloy  triturated  3  minutes  gave  5  microns  expansion  in 
24  hours.  This  same  alloy  gave  15  microns  contraction  when 
triturated  for  30  minutes.  A  similar  behavior  was  found  for 
other  alloys.  Overannealing  while  exposed  to  the  air  at  120°  C 
for  2  hours  in  an  electric  oven  produced  little  change  in  this 
chemical  expansion  or  contraction,  although  it  did  render  the 
alloy  slow  setting  and  of  granular  consistency  while  being  mulled. 
Balancing  an  alloy  then  becomes  meaningless  unless  proper 
instructions  are  given  for  time  of  trituration,  etc. 

In  this  investigation  comparatively  little  interest  was  paid  to 
the  manufacture  of  alloys  since  the  purpose  of  the  research  was 
to  test  the  finished  product.  No  effort  was  made  to  run  a  set  of 
definite  percentages  of  materials  to  recommend  the  proper  pro- 
portions for  a  perfect  alloy — the  companies  doubtless  prefer  to 
have  this  left  as  their  field.  The  properties  of  cleanliness,  size  of 
cut,  time  of  setting,  working  qualities,  annealing,  and  the  like 
must  all  be  incorporated  in  the  material  and  each  manufacturer 
will  endeavor  to  strike  an  average  such  that  the  alloy  will  contain 
a  maximum  of  desirable  qualities. 

Certain  desirable  or  undesirable  properties  are  supposed  to 
accompany  excessive  or  deficient  amounts  of  the  constituent 
metals,  for  example,  alloys  of  low  percentages  of  silver  are  sup- 
posed to  be  deficient  in  strength,  of  poor  working  qualities,  and 
slow  setting;  that  is,  tend  to  flow  badly.  These  facts  were  veri- 
fied qualitatively  only.     (See  Table  3.) 

High  percentages  of  copper  are  said  to  produce  discoloration — 
this  of  course  should  be  or  may  have  been  settled  by  the  dental 
profession.  However,  an  amalgam  made  from  an  alloy  contain- 
ing 16  per  cent  copper  was  discolored  when  exposed  to  a  weak 
solution  of  iodine,  very  much  more  than  any  of  four  others  con- 
taining a  lower  amount  of  copper. 

Zinc  has  long  been  considered  a  "disturbing"  element.  The 
claim  is  often  made  that  zinc  is  "  inadmissible,"  even  in  the  small- 
est amounts,  and  that  all  amalgams  containing  zinc  will  "  move." 
Since  this  movement  is  supposed  to  be  cumulative  and  to  extend 
over  a  period  of  years,  it  is  impossible  to  furnish  data  extending 
over  a  period  sufficiently  large  to  be  of  decisive  value. 


Physical  Properties  of  Dental  Materials 


37 


A  numlDer  of  specimens  have  been  prepared  and  are  under  ob- 
servation. These  have  been  divided  into  two  groups,  one  of  which 
is  kept  at  room  temperature,  the  other  is  subjected  to  tempera- 
ture variations  between  zero  and  50°  C.  At  the  end  of  the  fourth 
month  the  first  group  had  changed  in  length  by  values  ranging 
from  o  to  0.2  per  cent,  one  zinc  amalgam  showing  the  zero  change, 
another  zinc  showing  the  0.2  per  cent  change.  The  values  ob- 
tained by  averaging  the  changes  of  zincs  against  the  nonzincs, 
disregarding  signs,  indicate  a  more  nearly  permanent  state  for 
the  latter. 

The  group  subjected  to  temperature  variations  (o  to  50°  C) 
has  suffered  much  greater  changes.  Here  the  average  departures 
from  initial  length  appear  to  favor  the  zinc  amalgams,  the  aver- 
age departure  being  o.i  per  cent,  while  the  average  departure 
for  nonzinc  amalgams  is  0,2  per  cent.  The  extremes  of  the 
former  are  0.02  and  0.5  per  cent  and  of  the  latter  0.06  and  0.4 
per  cent. 

Manifestly  no  definite  conclusions  can  be  drawn  from  the  com- 
parison at  this  time.  At  a  later  date  it  will  be  possible  to  give 
more  conclusive  evidence  bearing  on  this  phase  of  the  problem. 

X.  ELECTRODE-POTENTIAL   DETERMINATIONS 

The  normal  calomel  half  cell  and  potentiometer  was  used  in 
measuring  the  potentials.  A  description  of  this  instrument  may 
be  found  in  any  text  book  of  physical  chemistry.  All  values 
except  the  last  are  for  the  resultant  amalgam  using  the  alloys 
specified.  A  number  of  amalgams,  ranging  in  values  from  o  to 
5  per  cent  zinc,  were  tested.  These  are  given  in  Table  2.  The 
potentials  are  expressed  in  terms  of  volts,  electromotive  force, 

TABLE  2 


Alloy 

Emf 

Alloy 

Eml 

-0.54 

-  .55 

-  .52 

-  .52 

-0.52 

0  per  cent  Zn  (duplicate) , 

-  .52 

-  .51 

0  per  cent  Zn,  16  per  cent  Cu  (duplicate). . 

Gold  (metallic) 

+  .002 

No  conclusive  inferences  are  to  be  drawn  from  these  although 
there  seems  to  be  little  evidence  of  excessive  contact  emf  effects 
due  to  the  larger  zinc  content. 

These  tests  seem  to  point  toward  a  complete  solution  of  the 
zinc  (at  least)  by  the  mercury.     This  finding  is  in  harmony  with 


38 


Technologic  Papers  of  the  Bureau  of  Standards 


the  practice  of  giving  zinc  electrodes  a  surface  amalgamation  to 
prevent  local  action,  due  to  impurities  when  placed  in  the  solu- 
tion of  the  electric  cell. 

TABLE  3. — Comparison  of  Amalgams 


Alloy 

Crushing 
strength 

in 
pounds 

per 

square 

inch 

Per 
cent 
flow 
in  24 
hours 

Rela- 
tive 
black- 
ening 

Setting  changes,  first 
24  hours  (all  values 
in     microns     per 
centimeter) 

Mark 

Partial 
composi- 
tion 

Maxi- 
mum 
con- 
trac- 
tion 

Maxi- 
mum 
ex- 
pan- 
sion 

Final 
state 

Claims 

A 

Ag68.... 
ZnO 

Ag67.... 
Zn2 

Ag68.... 
Znl 

Ag66.... 
Zn(l)... 

Ag60.... 
Zn(    ).. 

Ag66.... 
Znl 

Ag68.... 
Zn§ 

Ag67.... 
Znl 

Ag68.... 
Zn§ 

Ag67.... 
Znl 

Ag54.... 
ZnO 

Ag54.... 
ZnO 

Ag54.... 
Zn§ 

Ag45.... 
Zn2 

Ag67.... 
Znlg.... 

Ag60.... 
Zn5 

49  500 
51  550 

46  500 

40  300 

39  800 

38  200 
44  200 

39  950 
35  550 

41  950 
38  900 

49  500 
48  400 

47  250 
46  650 

40  750 

40  750 

38  700 

37  100 

42  750 
44  800 

41  550 

48  450 

33  600 

38  700 

30  400 

34  300 
30  300 

30  850 
38  500 
30  000 

40  750 
34  000 

41  200 

4.4 

7 

1.5 

5.6 

+  4.3 
+  4.5 
+  2.2 

+     .5 
-1.7- 

-  .2 

+  5.2 
+  6.7 
+  4.9 

+  2.1 
+  2.6 
+  3.7 

-6.8 
-5.7 
-5.0 

-27.1 
-21.3 
-20.2 

+     .4 

-  .3 
-1.4 

+  3.8 
+  3.5 
+     .7 

-7.5 
-6.5 

+  2.5 
+  2.4 
+  2.5 

+  1.4 
+  3.9 
+  4.2 

+  2.6 
+  3.0 
+  2.1 

—28.1 

A  correct  material  certified  as  to  balance 

B 

3.1 

1 

3.9 

1.5 

It  is  balanced,  produces  a  white  amal- 
gam   of    great    strength,    free    from 

C 

4.0 

0 

.6 

6.7 

slighest  contraction,  has  a  slight  initial 
expansion   never   exceeding    3/10  000 
inch,  takes  a  high  polish 
Made  in  accordance  with  the  investiga- 
tions of  Dr.  G.  V.  Black;  is  a  balanced 

alloy.    This  is  a  perfect  alloy  in  every 
respect,  is  age  proof,  an  exact  alloy, 
highest  crushing  strength.  Is  the  only 
chemically  clean  alloy  made 

D 

3.4 

0 

.8 

5.5 

E 

9.1 

4 

9.2 

.0 

White  alloy 

F 

10.4+ 

2 

27.1 

.0 

A  balanced  alloy  made  after  the  approved 
formula  of  G.  V.  Black,  M.  D.,  D.  D.  S. 

G 

5.0 

4 

3.5 

1.7 

Never  changes  color,  contraction  nil,  ex- 
pansion 1/20  000 

Used  by  the  U.  S.  Navy  made  after  the 
Black  formula 

H 

3.7 

1 

4.1 

4.3 

I 

4.3 

1 

7.7 

.0 

Does  not  shrink,  about  1/20  000  expansion 
takes  place;  a  perfect,  nonleaking 
amalgam  can  be  the  only  result 

A  balanced  alloy,  permanent  whiteness, 
made  by  the  Black  method,  contrac- 

J 

7.8 

8 

1.2 

3.8 

E 

9.6 

8 

2.4 

4.9 

tion  nil,  expansion  1/20  000 

Depended  upon  not  to  shrink  or  change 
form;    insures  against  stained  teetb 

L 

5.3 

4 

2.1 

3.2 

and  gutters  around  fillings 

Fillings  do  not  shrink  nor  change  form; 
has  a  fine  white  color  which  endures; 

M 

36.0 

10 

39.1 

.0 

can  not  shrink 

No  shrinkage,  no  discoloration,  a  tooth 
saver;  any  higher  price  can  not  buy  a 
better  alloy;  an  honest,  eflBcient  filling 
material;  takes  a  high  polish  and 
keeps  it 

Nonshrinkage,  nonexpansion,  edge 
strength,  retention  of  color 

N 

47.1 

7 

7.8 

.0 

-3.8 

O 

2.5 

2 

1.9 

.0 

-1.9 

P 

2.6 

1 

7.2 

.0 

-4.5 

1 

Physical  Properties  of  Dental  Materials  39 

Table  3  gives  a  brief  comparison  of  the  more  important  quali- 
ties investigated.  Some  of  these  are  readily  verifiable  by  the 
interested  dentist  who  is  willing  to  spend  the  money  necessary 
to  pm^chase  the  alloys.  Others  require  special  apparatus  or  the 
services  of  a  testing  laboratory,  for  example,  crushing  strength 
and  flow,  while  others  require  apparatus  of  such  high  precision 
that  in  addition  to  the  apparatus  it  is  essential  to  have  the  tests 
carried  out  by  a  person  skilled  in  the  use  of  such  apparatus,  for 
example,  accurate  determinations  of   expansion  and  contraction. 

The  claims  tabulated  are  those  made  on  the  label  of  the  package 
or  in  advertisements  placed  before  the  public  in  recent  years. 

It  is  a  regrettable  situation  that  individual  practitioners  will 
find  it  practically  impossible  to  make  complete  tests  on  the  ma- 
terials supplied  as  balanced  alloys  to  conform  to  methods  of  Dr. 
Black,  or  any  formula  or  specification.  However,  it  is  felt  that 
the  manufacturers  turning  out  products  which  are  of  a  question- 
able value  will  welcome  any  move  to  place  this  work  on  a  scien- 
tific basis,  thus  eliminating  the  necessity  for  lower  standards 
often  used  in  the  fields  where  price  is  put  before  permanence. 

There  may  possibly  exist  a  place  for  the  materials  of  question- 
able qualities — that  is,  tendency  to  flow  excessively  or  to  con- 
tract on  setting — which  necessitates  their  production  and  place  in 
the  market.  If  so,  specific  information  should  accompany  each 
package,  giving  full  details. 

XI.  SUMMARY 

A  survey  of  the  previous  work  on  the  physical  properties  of 
dental  materials  reveals  a  large  amount  of  qualitative  work  on 
certain  properties  with  instruments,  the  inherent  errors  or  sen- 
sitiveness of  which  are  comparable  with  the  magnitude  of  the 
effect  under  investigation. 

Many  of  the  essential  properties  have  not  been  considered  and 
in  some  cases  a  careless  interpretation  of  results  has  led  to  con- 
siderable confusion. 

Recent  work  by  Dr.  Gray,  of  Milford,  Del.,  with  improved 
apparatus,  has  indicated  the  possibilities  and  importance  of  ob- 
taining qualitative  results  on  more  of  the  properties.  At  the 
request  of  a  branch  of  the  Government  the  authors  undertook 
such  an  investigation. 

An  inspection  of  the  instruments  in  general  use  disclosed  their 
lack  of  sensitivity  and  necessitated  the  construction  and  employ- 
ment of  more  suitable  apparatus. 


40  Technologic  Papers  of  the  Bureau  of  Standards 

Accuracy  and  efficiency  recommended  the  use  of  the  inter- 
ferometer for  determinations  of  thermal  expansion  and  setting 
changes,  since  the  necessary  accuracy  can  be  secured  with  the 
use  of  small  specimens,  the  temperature  of  which  is  readily  con- 
trolled. 

For  crushing  strength  determinations  the  calibrated  testing 
machines  of  the  Bureau  were  used.  For  the  flow  tests  it  was 
found  desirable  to  select  a  special  instrument.  This  instrument 
consists  essentially  of  a  precision  dial  micrometer  equipped  for 
applying  constant,  continuous  pressures  to  specimens. 

The  electrode-potential  measurements  were  made  with  the 
calomel  half  cell  and  potentiometer,  which  is  standard  apparatus 
for  such  measurements. 

Careful  chemical  analyses  were  made  to  determine  the  con- 
stituents of  the  different  alloys. 

The  results  of  the  determinations  of  the  properties  tested,  which 
are  represented  in  the  accompanying  curves  and  tables,  show  the 
behavior  of  different  alloys  and  the  effects  of  different  conditions 
and  manipulations  upon  the  same  alloy. 

Because  of  otu:  lack  of  sufficient  clinical  experience  we  have 
not  attempted  to  speculate  on  our  data  except  under  definite 
physical  conditions,  but  have  attempted  to  emphasize  to  the  pro- 
fession some  of  the  important  properties,  together  with  a  means 
of  determining  the  same. 

We  are  pleased  to  make  the  following  acknowledgments  of 
assistance : 

Dr.  H.  D.  Holler,  Bureau  of  Standards,  for  measurements  of 
electromotive  force;  Miss  H.  C.  Baker  and  A.  M.  Weber,  Bureau 
of  Standards,  for  determining  crushing  strength  of  specimens; 
Mr.  J.  H.  Scherrer,  Bureau  of  Standards,  for  chemical  analyses; 
R.  I/.  Coleman,  Biureau  of  Standards,  for  amalgamating  numerous 
specimens,  etc.;  and  to  the  following  manufacttu-ers  cooperating: 
Atkinson  Laboratories,  L.  D.  Caulk  Co.,  Cleveland  Dental  Co., 
J.  M.  Ney  Co.,  and  S.  S.  White  Dental  Manufacturing  Co, 

Washington,  November  24,  1919. 


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